Automatic Reference Counting: Pointer to non-const

问题:

In updating some of my code to be in compatible with the iOS 5 SDK, I attempted to refactor my code by using "Convert to Objective-C ARC" in Xcode and received an error. The error occurs on an instance variable in my .h file.

NSError **_error;

The error says "Pointer to non-const type 'NSError *' with no explicit ownership." How might I fix this?

回答1:

When storing NSError objects in an instance variable you have to declare it as a simple pointer:

@interface Foo : NSObject
{
    NSError *_errror;
}

NSError ** is only used to indirectly return NSError objects from a method to the caller. It is (probably) an error to store it in an instance variable.

回答2:

You can use one of the lifetime qualifiers described in the Transitioning to ARC Release Notes

For NSError ** you'd use __autoreleasing so the example provided by Nikolai would then look like this

@interface Foo : NSObject
{
    NSError * __autoreleasing *error;
}

The syntax is a little weird and you should follow Nikolais advice, but if you're sure NSError ** is what you want this should work.